Engine light on

Hi I was wondering if anyone can help. I've got a 1999 Honda accord 1.8s and recently had a new exhaust down pipe fitted also the mechanic did some welding on the middle section but I found out that the battery was still connected. The problem is my engine light has come on and when at temperature the car idles really low and sometimes cut out completely. I naturally tried to use a diagnostic machine but found out this model doesnt have a diagnostic port which isn't very helpful. Some people are saying egr valve, oxygen sensor but the car has passed a emission test so please any ideas or solutions would be helpful.